Conventionally, a cold forging method is widely used as a method of producing a predetermined rotary member by forming, at the center of a plate-like metallic blank, only a case-like boss projecting from one lateral side of the blank, or as a method of producing a predetermined rotary member by forming, at the center of a plate-like metallic blank, a case-like boss projecting in one direction from one lateral side of the blank, and by forming, at the outer periphery of the blank, a case-like peripheral wall concentrically projecting in the same direction in which the boss projects.
The conventional cold forging method above-mentioned is adapted to form only a case-like boss, or both a case-like boss and a case-like peripheral wall with the use of a plastic flow of a material itself. There is a sheet metal product such as a rotor of an electromagnetic clutch or a V-pulley requiring a boss having a relatively small inner diameter and a relatively great projecting height, or a sheet metal product requiring a relatively narrow annular space between a boss and a peripheral wall which are concentrically formed at inner and outer portions of the product. When producing such a sheet metal product from a thick blank in order to assure a sufficient strength, it is difficult to form such a boss having a predetermined diameter and a predetermined projecting height, or to form such a narrow annular space, even though there is used a large-size press machine of the class of 2,000 tons to 2,500 tons. Accordingly, the range in which such a product can be formed, is automatically limited by the relationship between the sizes of diameter, height and the like, and the thickness of the blank.
In view of the foregoing, the present invention is proposed with the object of providing a method of producing a rotary member made of a metallic plate, capable of forming, with high precision, only a boss or both a boss and a peripheral wall each having a predetermined diameter, a predetermined thickness and a predetermined height, with the use of a small press machine without the thickness of the initial material reduced so much in the course of production steps.